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 MIL-HDBK-232A
5.1.1.2.3.1 Facility entrance plate. All facilities designed using this
handbook should use a facility entrance plate, whether or not a facility
shield is installed. All conductors entering the facility shall pass through
this plate. This plate is connected via a low impedance path to the low
impedance earth electrode subsystem (EESS). This plate provides an ideal
place to decouple compromising emanations from conductors egressing the
facility and provides a low impedance shunt for lightning and electromagnetic
pulse (EMP)/high-altitude electromagnetic pulse (HEMP) transients to earth to
prevent them from entering the facility.
5.1.1.2.3.2 Power entry. The power enters the facility as described in
paragraph 5.2. Terminal protective devices (TPDs), installed on all current-
carrying conductors, are installed to provide a low impedance path for
shunted current to earth via the facility entrance plate. This may often be
best accomplished by bonding the ground electrode of the TPD directly to the
facility entrance plate. The TPD must be installed in the power lines as
near as possible to the entry plate to minimize the lead length which may
carry high power transients (lightning or EMP) within the facility. Where
motor generators (MGs) with nonconducting shafts are installed, the technical
power is effectively isolated from conductors caressing the facility. An
uninterruptible power supply (UPS) may also provide isolation between the
source of power and technical power load, as well as isolating differing
classes of loads where a separate UPS serves each load. Installation of an
UPS also provides continuity of operation in the event of sustained
disruption of utility power.
5.1.1.2.3.3 Utility entrance. All utility pipes (water, fuel, etc.) shall
enter the facility through the facility entrance plate. Metal pipes are
circumferentially bonded to the plate. If the facility is shielded, plastic
pipes should pass through the plate via a waveguide-beyond-cutoff which is
circumferentially bonded to the plate. (A waveguide-beyond-cutoff is a
metallic pipe which has a length that is five times its diameter.) Any large
ducts, such as air ducts, which enter the facility are grounded at the entry
point and have waveguide-beyond-cutoff honeycomb installed across the entire
cross section of the duct at the point of entry. This is in addition to
physical security requirements.
5.1.1.2.3.4 Signal entry. All signal cables, whether used for data, timing,
control, telephone, or any other purpose, enter the facility through the
facility entrance plate. All cables should be shielded within the facility.
The shields of all cables passing through the facility entrance plate shall
be circumferentially bonded to the plate. (Shielded connectors are
permissible where the mating surfaces make a 360-degree contact and the
connector mounted on the plate is circumferentially bonded to the plate.)
The signal conductors passing through the plate are either filtered or
isolated, and protected by surge arrestors and other protective devices. It
is important that the shunting conductor of the filters and surge arrestors
have a low impedance path to the entry plate.
5.1.1.2.3.5 Facility ground system. The facility ground system serves to
attenuate both internally generated emanations and external disturbances, but
not to the extent that would be achieved by shielding the facility. The
ground resistance should not exceed 10 ohms. (Design objective,
MIL-STD-188-124.)
5.1.1.2.3.6 Earth electrode subsystem (EESS). The EESS provides an
essential part of the low impedance path to earth for shunting each
referenced disturbances such as power-line faults, lightning, and EMP/HEMP.
It typically consists of a ring ground around the facility, augmented by an
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